In hybrid electric vehicles (HEV) and electric vehicles (EV), in order to ensure a desired high voltage, a battery pack (battery system) in which a large number of unit cells in a secondary battery are connected in series with each other is used. In the battery pack thus configured, for the purpose of conducting capacity calculation or protection management of the respective unit cells, the unit cells are managed with the use of a monitoring IC for monitoring a state of the unit cells and a control IC for controlling a charge/discharge state of the unit cells (for example, refer to PTL 1).
In particular, in the battery system using a lithium ion battery, when the lithium ion battery is overcharged with a high energy density, there is a possibility that the lithium ion battery is damaged. For that reason, as disclosed in PTL 2, a voltage across each of the unit cells is measured by the respective control IC and monitoring IC to detect an overcharge state, and if any IC detects the overcharge, the charge/discharge of the battery is ceased, thereby enhancing the reliability and safety.
The monitoring IC detects the voltages across the unit cells, individually, and for example, if there is a unit cell of the overcharge state, the monitoring IC transmits overcharge information to the control IC through a communication. A test signal is transmitted from the control IC to diagnose whether an abnormality such as disconnection occurs in a communication line, or not, so that the overcharge information is surely transmitted to the control IC.
In detecting the voltages across the unit cells, a given unit cell is selected by a multiplexer, and the voltage is detected by a voltage detection unit. A connection of the multiplexer is switched to others, thereby being capable of detecting the voltages across all of the unit cells. In order to acquire correct cell voltages across the respective unit cells, a cell voltage measurement system including the multiplexer is required to correctly operate. For that reason, in the invention disclosed in PTL 3, a sum of cell voltage measured values of the respective unit cells is compared with a total cell voltage measured value measured by a total voltage measurement system, and if those values are remarkably different from each other, it is determined that the failure occurs in the cell voltage measurement system including the multiplexer.
If a voltage detection line is not normally connected to each unit cell, the voltage across the unit cell cannot be normally detected. In the invention disclosed in PTL 4, the cell voltages across the unit cells are measured while operating a balancing circuit intended to uniformize the charge state of the unit cells, and operation for detecting disconnection is automatically conducted within an integrated circuit on the basis of the cell voltages.
In recent years, a demand to speed up a diagnosis of the voltage measurement system including the disconnection of the voltage detection lines has been increased. For example, if the multiplexer is diagnosed as described above, the multiplexer is diagnosed on the basis of the measured voltage between terminals of each unit cell up to now.
Since the inter-terminal voltage of the unit cell is originally measured to determine the overcharge or overdischarge state of each unit cell, there is a need to measure the inter-terminal voltage with precision. For that reason, a voltage measurement circuit having a resistance to noise is used for measuring the inter-terminal voltage. For example, there are a voltage measurement circuit using a double integrating type AD converter as disclosed in PTL 1, and a voltage measurement circuit to which a voltage is input after the removal of noise with the configuration of an RC filter having an input capacitor disposed between the voltage detection lines as disclosed in PTL 4.
This voltage measurement circuit resistant to the noise takes a long measurement time for removing a noise component, as a result of which it also takes time to diagnose the disconnection of the voltage detection lines or the voltage measurement system such as the multiplexer, which is conducted by the aid of the voltage measurement.
Also, as in the invention disclosed in PTL 3, when a failure of the cell voltage measurement system is determined by comparing the cell voltage measured value of the unit cell selected by the multiplexer with the total voltage of the battery, there arises the following problems. For example, in a state where the multiplexer breaks down to always select only a specific unit cell, if the charge states of the respective unit cells are substantially equal to each other, there is substantially no difference between the summed voltage and the total voltage. This leads to a possibility that even if the voltage detection line of the cell other than the selected unit cell is disconnected, no abnormality is detected, and the voltage detection system including the multiplexer is determined as normality. Also, when the unit cells are selected by the multiplexer, even if the multiplexer operates normally, the summed value of the cell voltages and the total voltage may be different from each other in the interest of synchronism of the cell voltage measurement if a voltage fluctuation is steep, resulting in a possibility of erroneous determination. Therefore, in order to surely detect the overcharge, there is a need to separate the monitoring IC and the control IC from each other to enhance the reliability as described above.
Also, in PTL 4, in the hybrid electric vehicle (HEV) and the electric vehicle (EV), for the purpose of ensuring a desired high voltage, the battery pack (battery system) in which a large number of unit cells in the secondary battery are connected in series with each other is used. In the battery pack of this type, for the purpose of conducting the capacity calculation or protection management of the respective unit cells, an integrated circuit that measures and equalizes the cell voltages is used within a monitoring apparatus of the battery pack to manage the unit cells. In order to enhance the reliability, the integrated circuit not only measures and equalize the cell voltage, but also conducts a variety of diagnoses including the disconnection detection of the cell voltage detection lines to enhance the reliability.
The monitoring apparatus of this battery pack always monitors the total voltage and charge/discharge currents of the battery pack, calculates a state of charge (SOC) and a direct current resistance (DCR) as the battery pack, transfers a maximum input/output power or current as the battery pack from that value to a high-level controller, and the high-level controller controls a charge/discharge power (current) from that value. For that reason, the total voltage detection of the battery pack is essential, and a variety of total voltage detector circuits have been proposed (for example, refer to PTL 5).
Because an output of the battery pack configured by a large number of unit cells is high voltage, the battery pack of the high voltage is isolated from a chassis (GND) of a vehicle in the total voltage measurement circuit of this type. Also, a dedicated voltage measurement circuit using a voltage divider circuit of high resistance is employed.
Also, a communication of a battery monitoring integrated circuit and the high-level controller is also isolated, and a photocoupler is generally used as an insulating element for isolation. In order to conduct the communication through the photocoupler at a high speed, a flowing current at a light emitting diode side needs to increase, and its power supply is supplied from the battery pack, and cannot be increased greatly. For that reason, when the photocoupler is used, a communication system is divided, and the communication is conducted at a slightly low speed.
In recent years, the battery pack formed of the secondary battery such as the lithium battery is remarkably increasingly used in the electric vehicle (EV) or a plug-in hybrid vehicle (PHV). When the secondary battery is used in the EV or the PHV, because a use range of the SOC of the secondary battery is wider than that when the secondary battery is used in the hybrid electric vehicle (HEV), also because a fluctuation of the SOC due to a large current output is quick, there is required to voltage-monitor and diagnose all of the cells at a high speed.
The normal battery system includes a plurality of cell groups each having a plurality of unit cells connected in series, and the battery monitoring integrated circuit is disposed for each of the cell groups. Therefore, in order to voltage-monitor and diagnose all of the cells at a high speed, high-speed communication means between the battery monitoring integrated circuit and the high-level controller is required.
Also, the number of unit cells connected in series in the battery pack is increased. In the cell group where the plurality of unit cells are connected in series, which is a unit of the secondary battery which is monitored by the battery monitoring integrated circuit, for example, ten or more unit cells are connected to each other, and an inter-terminal voltage of the cell groups is several tens or more volts. Because the inter-terminal voltage of the cell groups becomes high voltage, a problem of a dark current in the battery monitoring integrated circuit when the vehicle operation stops becomes actualized.
Further, since a capacity of the unit cells is increased, the monitoring and control of the battery system corresponding to this increased capacity is necessary.